1. Field of the Invention
The invention relates generally to arc lamps and more particularly to short arc lamps with improved structural and mechanical integrity, and improved light output.
2. Description of the Prior Art
In optical systems involving the generation and controlled radiation of long or continuous pulses of light, such as spectroscopy, or solar simulation, where high intensity, color correct illumination of sensitive working areas is required, such as in fiber optics illumination devices, it is advantageous to have a light source capable of producing the highest possible light flux density. Products utilized in such applications include short arc inert gas lamps, comprising a sealed chamber containing a gas pressurized to several atmospheres, and an opposed anode and cathode defining an arc gap. A window provides for the transmission of the generated light, and a reflector may be positioned surrounding the arc gap.
U.S. Pat. No. 3,502,929 issued to Richter discloses a high power, high output, short arc lamp utilizing a ceramic chamber sealed with a sapphire window. Richter discloses a cathode placed adjacent to the ceramic chamber and an anode suspended adjacent to the window. Because of heat generation at the anode during operation, the anode support structure, seals and window of Richter are relatively thin to dissipate the heat. At operational temperatures, the pressure inside the chamber may increase from approximately seventeen atmospheres to approximately thirty atmospheres. At this elevated pressure the lamp of Richter suffers from mechanical stress resulting in gas leakage and cracking of the window.
An attempt to solve these problems is disclosed in U.S. Pat. No. 3,731,133 issued to McRae et al., wherein the cathode and anode positions are reversed to provide the anode with a massive base for heat dissipation. While cracking due to heat was reduced, the problem of maintaining the integrity of the chamber under elevated pressure conditions remains. In particular, the window assembly of McRae et al. is secured to the ceramic cylinder by means of two J-shaped window flanges, placed one atop the other. These are in turn secured to a single thin back-up ring, brazed to the top of the ceramic cylinder.
It has been found that the single thin back-up ring may be insufficient to properly secure the window at operational pressures. Further the use of multiple components in the window assembly requires multiple seals, thus increasing the likelihood of leakage.
Other prior art attempts to alleviate the leakage, stress, and window cracking problems include that disclosed in U.S. Pat. No. 3,808,496 issued to McRae et al. This patent utilizes the teachings of McRae et al. '133 and places the anode in the lamp base and further provides a compression mounting for the window. U.S. Pat. No. 3,852,629 issued to Stuart discloses a window mounting using a single J-shaped flange.
Reducing the gas pressure in the lamps typically cures the leakage, stress, and cracking problems but photometric output is reduced and the voltage characteristics of the lamp change, reducing lumenous flux and brightness of the lamp. Different materials may also be employed in the lamp, for example, a quartz window, however these require a much more costly series of graded materials to effect a proper seal.
An additional problem with prior art lamps having a pressed ceramic cylinder with an integral reflective surface is that the reflector surface typically varies by a significant amount from the desired shape. Because the point of highest light intensity in the arc is very small, it is important that the focal point of the reflective surface coincide as closely as possible with this point to ensure the greatest light flux density. While much art, including McRae et al., are addressed to the problem of positioning the reflector relative to the cathode, there remains a need for improvements in the production of the reflector surface itself, whereby it may more closely conform to the desired curve, typically either parabolic or elliptical.
Integral reflector surfaces may be formed in the ceramic envelopes by pressing into a hot ceramic cylinder a mandrel having the desired shape. Because the reflector surface must be highly polished, the mandrel is polished to impart smoothness to the ceramic surface. Polishing a mandrel, however, is costly and inconsistent and tends to remove sufficient material from the mandrel to significantly alter the desired reflector shape. While mandrels may be polished to compensate for this, the methods to accomplish such compensated polishing are very costly and manufacturers tend to sacrifice accurate reflector shape for lower cost. The situation is further complicated by the tendency of ceramic to shrink as it cools during the firing process.
In view of the prior art, there remains a need for a short-arc lamp having improved leak and cracking resistance and with improved light output at nominal cost.